The present invention relates to polychloroprene rubber compositions modified with elemental sulfur.
Various modifiers containing sulfur are used to produce different types of polychloroprene, each type having somewhat different and desirable properties from the other types. For example, polychloroprene sulfur modifiers include elemental sulfur, mercaptans, xanthogens and polysulfides. The types containing elemental sulfur cure rapidly without acceleration, and with milling they break down to a greater extent than other types which is useful in achieving workable viscosity in highly loaded stocks with a minimum of plasticizer.
The polychloroprene prepared with elemental sulfur also is desirable since it has less shrinkage than the other types and sheets out better.
Vulcanizates of the elemental sulfur modified polychloroprene are superior in tear strength, resilience, elongation and adhesion to natural rubber and SBR to other polychloroprenes.
A problem, however, with the elemental sulfur modified polychloroprenes has been the lack of stability of the raw polymer. This has resulted in significant loss of Mooney viscosity of these compounds during storage.
Polychloroprene blends, having improved processing characteristics, have been developed by blending polychloroprenes which are benzene-soluble, referred to as "sols", with polychloroprenes which are benzene-insoluble, referred to as "gels". The resulting blends are frequently referred to as "easily processable" blends. Examples of these blends may be found in U.S. Pat. Nos. 3,042,652; 3,147,317; 3,147,318; and U.S. Pat. No. 3,655,827, which disclosed that superior blends are prepared from sols having dialkyl xanthogen disulfides as modifying or chain-transfer agents and gels having cross-linking monomers containing two or more polymerizable double bonds.
The examples of U.S. Pat. No. 3,655,827, issued to Joseph B. Finley et al, demonstrated the substantial superiority of the easily processable rubbers, wherein the sol contained the dialkyl xanthogen disulfides and the gels contained ethylene dimethacrylate as compared to easily processable blends, wherein the sol was made with dodecyl mercaptan and the gel was made with the ethylene dimethacrylate. Thus, in order to take advantage of the invention as disclosed and taught by Finley et al, one must use the dialkyl xanthogen disulfides and thereby forego those sol polymers produced, for example, with the older mercaptans. In commonly assigned copending application Ser. No. 498,067, filed Aug. 16, 1974, it was disclosed that a copolymerizable monomer comprising a triester of a triol and unsaturated acids used in the gel will provide the same degree of benefit as Finley et al observed, but without the necessity of using a dialkyl xanthogen disulfide as Finley et al.
It is a feature of the present invention that the advantages of polychloroprene modified with elemental sulfur are obtained with improved raw polymer stability, particularly, Mooney viscosity. Surprisingly, an excellent blend rubber based on a sulfur modified sol was discovered.